This protocol is a randomized, double-blind, placebo-controlled, multi-center, parallel design clinical trial of the safety and efficacy of azimilide in patients with history of symptomatic atrial fibrillation, atrial flutter or paroxysmal supraventricular tachycardia. Patients must, in the judgment of their physician, require treatment. Patients will be stratified according to one of two index arrhythmias. One stratum will include patients with atrial fibrillation or flutter; the other will include patients with paroxysmal supraventricular tachycardia. Study subjects will be randomly assigned to receive daily oral doses of 100 mg of azimilide or placebo. Patients will be assigned to treatment in a ratio of two patients in the active treatment group to one patient in the placebo group. At least 234 patients with an index arrhythmia of atrial fibrillation or flutter will be enrolled in the trial. Although not a requirement for trial completion, the goal is to recruit 54 patients with an index arrhythmia of paroxysmal supraventricular tachycardia. The trial will consist of a three-day loading period followed by an efficacy period of 270 days (approximately 9 months). Patients will take their trial medication twice daily for the first three days and once daily thereafter.